Decreasing world reserves and diminishing availability of crude oil have created considerable incentive for the development and use of alternative fuels. In recent years, the ever increasing value of fossil hydrocarbon liquids and gases has directed research and development to the possibilities of employing bio-mass materials for fuel purposes. In particular, attention has been focused on fermentation derived ethanol for car fuel purposes. Ethanol is gaining wide popularity as such a fuel. Ethanol can be combined with gasoline to form a mixture known as gasohol. Automobiles can run on gasohol containing up to about 20 volume percent ethanol without requiring engine modifications. To prevent phase separation during storage, gasohol should be essentially free of water. Therefore ethanol used for gasohol production is preferably at least 199 proof.
Ethanol is derived primarily from the fermentation of mash, usually from corn, grains, and/or sugar cane. During alcoholic fermentation, sugar, particularly glucose, is converted into ethanol and carbon dioxide in the presence of yeast cells that contain the enzyme complex zymase. Glucose is produced by enzymatic splitting of maltose, which is itself formed during the hydrolytic, enzymatic splitting of starch or is developed during the manufacture of sugar. In addition to ethanol, the ethanol solutions developed during alcoholic fermentation contain soluble and insoluble components of vegetable cells and builders, yeast cells, starch and fractions of starch, various sugars, salts and water. The ethanol content of the solutions obtained during alcoholic fermentation is usually about 12 wt. %, since a higher ethanol concentration becomes toxic to yeast and larger amounts of other metabolic chemicals are formed. At higher alcohol levels, the yeast die and fermentation ceases.
As described in the online Encyclopaedia Brittanica, rectification is the process of purifying alcohol by repeatedly or fractionally distilling it to remove water and undesirable compounds. A fermentation mixture primarily contains water, ethanol, solids, and yeast. Distillation involves increasing the percentage of ethanol in the mixture. The fermentation mixture furthermore contains small quantities of constituents such as, for example, organic aldehydes, acids, esters, and higher alcohols. The ones that remain in the product are called congeners, and the congener level is controlled by the particular rectification system and by the system's method of operation.
A multicolumn rectifying system commonly consisting of three to five columns had been used in earlier years. The first column is a preliminary separation column called the beer still, or analyzer. It usually consists of a series of metal plates with holes punched in them and baffles to control the liquid levels on the plates. The product coming from this column is generally between 55 and 80 percent ethanol. A 95-percent product can be produced on a two-column system consisting of a beer column and a rectifying column with further purification added in the additional columns.
Water cannot be completely removed from ethanol by distillation because of the formation of an azeotrope containing 95.5 wt. % ethanol and 4.5 wt. % water, which limits the upper concentration of ethanol that can be obtained by rectification regardless of the number of theoretical plates employed. Distillation processes have the further drawback that they require a large amount of energy. Special techniques are required to dehydrate ethanol beyond the 95.5 wt. % ethanol content level and typically require a considerable additional amount of energy. The high energy cost of ethanol separation by distillation is an economic impediment for using ethanol produced by alcoholic fermentation as an engine fuel.
Alcohols other than ethanol are also being considered for bio-fuel applications. For example, 1-butanol can be obtained by a fermentation process and used for this purpose. Thus, useful means of separating alcohols such as 1-butanol from water are also desirable.
Azeotropic or extraction distillation based separation processes are based on the addition of an entrainer to the ethanol-water system. Traditionally, distillation with a third component has been used to form a minimum azeotrope. This technique lowers the boiling point below 78.15° C., the boiling point of the ethanol-water azeotrope. This can be a binary azeotrope such as water-ethyl ether, reported by Othmer & Wentworth, Ind. & Engr. Chem., 32, 1588 (1940), or a ternary azeotrope such as benzene-water-ethanol, well described in Kirk & Othmer, Encyclopedia of Chemical Technology. While the benzene-water-ethanol ternary is probably the most widely used method of dehydrating ethanol, it require the expenditure of a great deal of heat energy.
Extractive distillation involves the distillation of ethanol-water mixtures in the presence of an added solvent. The ethanol-water mixture is fed to a tray located in the intermediate part of the column, and the solvent is fed to a higher tray. The distillate contains water, with reduced amounts of ethanol; the bottom product contains solvent and ethanol with minimal amount of water. Subsequent distillation of the bottom product produces a second overhead with high concentration of ethanol. Both distillation steps may be conducted at atmospheric pressure, but it is preferred to operate the second distillation step below atmospheric pressure, for example, between 1 and 50 kPa.
Extractive distillation to remove water from ethanol is described in U.S. Pat. No. 1,469,447, where glycerin is used as the extractive agent; U.S. Pat. No. 2,559,519, where ethoxyethanol and butoxyethanol are employed; and U.S. Pat. No. 2,591,672, which discloses gasoline as being effective. Also, French Patent No. 1,020,351 describes the use of glycols, glycol ethers or glycol esters as extractive agents, and U.S. Pat. No. 2,591,671 reports the use of butyl, amyl and hexyl alcohols for this purpose. U.S. Pat. No. 2,901,404 suggests sulfuric acid, acetone or furfural as extractive distillation agents, and U.S. Pat. No. 4,349,416 teaches the use of ethylene glycol. U.S. Pat. No. 4,366,032 discloses ethanolamine and N-methyl pyrrolidone as effective extraction agents, and U.S. Pat. No. 4,400,241 reports the use of alkali-metal or alkaline-earth metal salts, sodium tetraborate dissolved in ethylene glycol, and dipotassium phosphate dissolved in glycerol. U.S. Pat. Nos. 4,428,798 and 4,455,198 propose the use of 2-phenyl phenol, cumyl phenol, diisopropyl phenol, cyclohexyl cyclohexanone, phenyl cyclohexanone, and cyclohexyl cyclohexanol as extraction agents. The disclosures of all of these patents are incorporated herein by reference.
Design and economic studies by Black at al., Chem. Eng. Progr., 50, 403 (1980), Ind. Eng. Chem., 50, 403 (1958)) show that n-pentane is a good entrainer for removal of water from ethanol. Like benzene or toluene, n-pentane forms a minimum boiling heterogeneous ternary azeotrope with ethanol and water. The ethanol product is withdrawn at the bottom of the azeotropic column. No phase splitting occurs in the columns, but two liquid phases of different compositions are formed in the decanter. The light phase contains 95% pentane, and the heavy phase contains 90% of water. The pentane in the light phase is returned to the column as reflux, and the heavy phase is concentrated using a two stage column.
Adsorption based separation processes for the removal of water from alcohols are described in, for example, U.S. Pat. No. 2,137,605; German Patent No. 1,272,293; and Canadian Patent No. 498,587, which collectively describe the use of either adsorbents or absorbents, including materials such as alumina and zeolites, for drying ethanol. The disclosures of these patents are incorporated herein by reference.
U.S. Pat. No. 4,277,635 describes the use of a crystalline silica polymorph (silicalite) for the adsorption of ethanol from an aqueous ethanol mixture, followed by recovery of the adsorbed, dehydrated ethanol by passing carbon dioxide gas through the silicalite bed. U.S. Pat. No. 4,273,621 describes a gas phase distillation dehydration process using crystalline zeolite molecular sieves, and a carbon dioxide gas stream as a drying aid. This reference teaches that zeolite sieves having a pore diameter of three Angstroms are useful; other adsorbents such as molecular sieves, carbon, alumina and silica would, in addition to adsorbing water; co-adsorb the ethanol and the carbon dioxide drying aid. The disclosures of these patents are incorporated herein by reference.
Also, materials such as fresh quicklime, anhydrous calcium chloride, anhydrous calcium sulfate, fused anhydrous potassium acetate, sodium acetate, barium oxide, silica gel, and various zeolites have been widely employed, silica gel and zeolites probably being the most commonly used. All of these reagents have disadvantages in that they must be extensively treated to remove the water before they can be reused.
Zeolite molecular sieves are adequate adsorbents for the removal of small amounts of water from organic solvents. By virtue of their small diameter (0.28 nm), the water molecules can easily penetrate the structural zeolite canals, while many organic molecules such as ethanol (0.44 nm), are excluded. The use of zeolites to remove water from ethanol is described in, for example, papers by Carton et al. (1987), Sowerby and Crittenden (1988), Ruthven (1984), Teo and Ruthven (Ind. Eng. Chem. Process Des. Devel., 5, 17 (1986)), and Carmo and Gubulin (Latin American Applied Research, 31, 353 (2001)).
U.S. Pat. No. 4,345,973, the disclosure of which is incorporated herein by reference, proposes a method for dehydration and/or enrichment of aqueous alcohol mixtures wherein the mixtures in the vapor state are contacted with a dehydration agent composed of cellulose, caboxymethylcellulose, cornmeal, cracked corn, corn cobs, wheat straw, bagasse, starch, hemicellulose, wood chips, other grains, other agricultural residues or mixtures thereof.
Salt distillation processes are described in, for example, U.S. Pat. No. 1,474,216, which teaches the extractive distillation of ethanol from water using solutions of calcium chloride, zinc chloride, or potassium carbonate in glycerol. The vapor pressure of the dissolved salt is so low that it never enters the vapor phase. The disclosure of this patent is incorporated herein by reference.
Also, Johnson and Further (Can. J. Chem. Eng., 43, 356 (1965)) reports that even a low concentration of potassium acetate eliminates the azeotropic behavior of the mixture. Rather then using a solvent that contains the dissolved salt, the salt can be added as a solid or melt directly into the column by dissolving it in the liquid reflux before it enters the column. At salt concentrations below the saturation point, almost pure ethanol can reportedly be achieved.
Salt distillation is accompanied by several problems, the most important of which is corrosion. Salt distillation columns require stainless steel or alloyed corrosion-resistant materials. Feeding and dissolving the salt also represents a potential problem; the solubility of salt is low in the reflux because it contains the more volatile component (ethanol), while the salt will be most soluble in the less volatile component accumulated at bottom. The presence of salts may increase the potential for foaming and possibility of salt crystallization in the column.
Detailed discussion of anhydrous ethanol production from a diluted aqueous solution of ethanol via extractive distillation with potassium acetate is discussed in Ligero et al., Chemical Engineering and Processing, 42(7), 543-552 (2003). In the first of two process flow sheets, diluted ethanol is directly fed to a salt extractive distillation column, and the salt is recovered in a multiple effect evaporator followed by a spray dryer. In the second flow-sheet, the concentrated ethanol from conventional distillation is fed to a salt extractive distillation column. In this case, salt is recovered in a single spray dryer. In both processes the recovered salt is recycled, the second process requiring less energy than the first.
U.S. Pat. No. 4,492,808, the disclosure of which is incorporated herein by reference, describes an extraction-based process for separating ethanol from an aqueous solution, wherein an ethanol-containing solution is extracted with CO2, C2H4 or C2H6 in the form of liquids or supercritical gases. If CO2 is used as the extracting agent, the extraction can take place at 30-150 atmospheres and at 0-150° C. In this process, the pressure of the ethanol-containing extract phase is reduced, and the ethanol is separated from the extraction agent by distillation. The ethanol, after the yeast is removed, is heated to 75° C., compressed by a pump to a pressure of 80 bar, and conveyed into mass transfer column, where it is contacted in countercurrent flow with the ascending supercritical CO2 phase, causing the ethanol to transfer from the aqueous phase to the supercritical CO2. At the bottom of the mass transfer column, the solution, which has an ethanol concentration of 2.2 wt. %, is removed. The pressure is reduced to atmospheric pressure, and the solution is degassed and returned to the alcohol fermenter.
The ethanol-containing solvent phase is fed from the top of the mass transfer column into the adsorber vessel and, at 75° C. and 80 bars, is exposed to granulated activated carbon. The ethanol is adsorbed by the activated carbon while the CO2 is returned, with the aid of a compressor, to the bottom of the mass transfer column. The regeneration of 1 kg activated carbon requires 7.1 kg CO2. Energy consumption in this example is about 4500 kJ/kg of product.
A distillation with chemical reaction process is a unit operation in which a chemical reaction and distillation are carried out simultaneously. Reactive distillation combines a chemical reactor and a distillation column in a single unit. Faitakis and Chuang (Chem. Eng. Commun., 192, 1541 (2005), and Ind. Eng. Chem. Res., 43, 762 (2004)) discuss the application of catalytic distillation to the dewatering of ethanol.
Catalytic distillation is a specific modification of reactive distillation and can be defined as a process in which heterogeneously catalyzed chemical reaction and separation occurs simultaneously in a single distillation column. Water is removed from ethanol by reaction with olefins, isobutylene being suitable for this purpose. The product of the reaction, t-butyl alcohol, must be removed from the ethanol by distillation. Unfortunately, water can be completely removed only by using large excess of isobutylene, and some of the ethanol is converted to its t-butyl ether.
Membrane systems have been employed to separate mixtures of miscible liquids by reverse osmosis. In such a process, the charge liquid is brought into contact with a membrane film, and one component of the charge liquid preferentially permeates the membrane. The permeate is then recovered as a liquid from the downstream side of the film. U.S. Pat. No. 5,139,677, the disclosure of which is incorporated herein by reference, describes the use of membranes with solutions containing 95 wt. % ethyl alcohol to recover product containing decreased quantities of water.
U.S. Pat. No. 5,028,240, the disclosure of which is incorporated herein by reference, describes the use of a freezing technique to purify ethanol. Most of the water from a dilute aqueous solution is removed by chilling sufficient to enable water to be separated in the form of ice crystals. Simultaneously, the remaining liquid is extracted at the same low temperature with a liquid organic solvent that is substantially immiscible with water but has high affinity to ethanol, causing alcohol to transfer to the organic phase. Ethanol separated from water and concentrated in an organic solvent such as toluene is useful as an addition to gasoline.
Water gas-shift reactions are employed in many industrial processes, including ammonia synthesis and hydrogen production. A water-gas shift reaction is a reversible, exothermic chemical reaction, frequently assisted by a catalyst, whereby steam reacts with carbon monoxide to produce carbon dioxide and hydrogen gas as shown below.H2O(g)+CO(g)⇄CO2(g)+H2(g) ΔH=−41.2 kJ/mol.
The water-gas shift reaction may actually occur in two reversible steps involving initial formation of formic acid from carbon monoxide and water. In a second step, the formic acid formed decomposes to hydrogen and carbon dioxide.CO+H2O⇄HCO2HHCO2H⇄H2+CO2 
Many materials are capable of catalyzing the water-gas shift reaction, but two classes of materials used almost exclusively in the industry as shift catalysts are iron based catalysts and copper-zinc based catalysts.
Iron based catalysts are high-temperature catalysts, operating at temperatures of about 320-450° C. Iron oxide catalysts can tolerate small quantities of sulfur and are fairly rugged. Copper-zinc based catalysts are low-temperature catalysts that operate at temperatures of about 200-300° C. These catalysts have good activity at low temperatures and are attractive because the reaction equilibrium is more favorable at low temperatures. In addition to exhibiting high activity, low-temperature shift catalysts are very selective, with minimal side reactions.
Copper-zinc shift catalysts are extremely sulfur intolerant, being irreversibly poisoned even with small quantities of sulfur compounds. Guard beds are often used to reduce the sulfur level in the feed stream. The low temperature catalysts can be also irreversibly damaged by temperatures above 360° C.
W. Ruettinger, O. Ilinich, R. J. Farrauto, J. Power Sources, 118, 61-65 (2003) describe Selectra Shift™, developed by Engelhard Corporation, as an alternative to commercial CuZn catalyst for carrying out the water-gas shift reaction.
Another material that has received attention as an industrial water-gas shift catalyst is sulfided cobalt oxide-molybdenum oxide on alumina. This type of catalyst is completely insensitive to sulfur poisoning and possesses good activity even at low temperatures.
Water-gas shift catalysts are discussed in, for example, Cai et al., U.S. Pat. No. 6,627,572, the disclosure of which is incorporated herein by reference. Preferred water-gas shift catalysts include catalysts containing copper oxide, zinc oxide, aluminum oxide, and combinations thereof. A typical low temperature shift catalyst is reported to contain from about 30% to about 70% CuO, from about 20% to about 50% ZnO and from about 5% to about 40% Al2O3.
Industrial shift reactors are typically multistage adiabatic reactors with cooling between stages. The cooling assists the reaction to be closer to an optimum reaction path; heat exchangers or injection of condensate can be used for heat removal between individual stages. In the industry, configurations with three beds are commonly used, with the two top layers containing a high-temperature catalyst and a lower third bed containing a low temperature catalyst to complete the reaction.
U.S. Pat. No. 6,387,554, the disclosure of which is incorporated herein by reference, describes a process for the production of hydrogen and electrical energy from ethanol. The process is characterized by the partial oxidation/reforming of ethanol with water for hydrogen production which is subsequently fed to a fuel cell for production of electrical energy. The use of the water-gas shift reaction is reported as a means of removing carbon monoxide from the hydrogen formed, since excess carbon monoxide can interfere with the functioning of fuel cells. However, the process described in U.S. Pat. No. 6,387,554 does not produce dry ethanol.
Despite the technology described above, there remains a need to develop new methods to be able to economically separate water from alcohol on a large-scale. In particular, there is a need to separate water from ethanol on a large-scale and to produce ethanol that is nearly free of water.